Preparation of addition product of lithium aluminum hydride and a tertiary alkyl amine



1960 J. 5. ROSCOE ETAL 2,955,126 PREPARATION OFADDITION PRODUCT OF LIT HIUM ALUMINUM HYDRIDE AND A TERTIARY ALKYL AMINE Original Filed June 15, 1955 George W. Schoeffer John S.Roscoe INVENTORS ATTORNEYS United States atent Q PREPARATION OF ADDITION PRODUCT OF LITHIUM ALUMINUM HYDRIDE AND A TERTIARY ALKYL AMINE Original application June 15, 1955, Ser. No. 515,726,

now Patent No. 2,867,498, dated Jan. 6, 1959. Divided and application May 14, 1958, Ser. No. 735,234

9 Claims. (Cl. 260-448) Our invention relates to the production of solid addition products of lithium aluminum hydride and lower tertiary alkyl amines. Our invention relates further to the use of the novel addition products, by thermal decomposition thereof, to eifect purification of lithium aluminum hydride and lower tertiary alkyl amines. This application is a division of our copending application Serial No. 515,726, filed June 15, 1955, now Patent No. 2,867,498.

Lower tertiary alkyl amines, such as trimethyl amine, frequently have associated with them impurities, such as water, ammonia, primary amines or secondary amines. In one aspect, our invention provides a means whereby lower tertiary alkyl amines which have one or more of the aforementioned impurities in admixture with them can be obtained in a purer form. This purification can be accomplished in accordance with this aspect of our invention by reacting the lower tertiary alkyl amine while the impurity or impurities are in admixture with it with sutficient lithium aluminum hydride to form a 1:1 molar addition product of the lower tertiary alkyl amine and the lithium aluminum hydride in solid form. A sufficient excess of lithium aluminum hydride is provided to react with all of the impurities in the tertiary amine and to furnish, in addition, one mole of lithium aluminum hydride for each mole of tertiary amine present in the impure amine charged. A further excess of lithium aluminum hydride is not harmful and some may be added to insure that ample is present. However, further excess appears unnecessary.

The addition product can be formed in various ways, for example, by contacting the gaseous lower tertiary alkyl amine with the solid lithium aluminum hydride or by passing the lower tertiary alkyl amine in gaseous form into a solution of the lithium aluminum hydride in a suitable dialkyl ether. When the latter expedient is used, the addition product formed is insoluble in the ether and separates therefrom in the form of a precipitate. The addition product which is formed is readily separated into its original lithium aluminum hydride and lower tertiary alkyl amine components by simple warming. Thus, the addition product is a convenient source of the lower tertiary alkyl amine in a highly pure form. The impurities in the charged tertiary amine may form other reaction products soluble or insoluble in ether but none of these appear to be products which subsequently can regenerate by thermal treatment any tertiary amine or volatile materials to contaminate it. Thus by evacuation of any volatile by-products at a low temperature and subsequent warming of the residual addition'product, substantially pure tertiary amine is obtained.

In another aspect, our invention provides improved means whereby lithium aluminum hydride can be purified ride in the known manner. The usual impurities in 2,955,125 I Patented Oct. 4, 1960 2 lithium aluminum hydride include aluminum hydride, lithium hydride, aluminum chloride, aluminum metal and lithium metal. Ether extraction of the impure product is known toeliminate all of these impurities except alu minum hydride which dissolves with lithium aluminum hydride in the ether. Both are recovered together from the ether solution. To eliminate the aluminum hydride according to this aspect of our invention, the mixture of lithium aluminum hydride and aluminum hydride in solution in a lower dialkyl ether is reacted with a sufficient amount of a lower tertiary alkyl amine to convert both the hydrides to 1:1 addition compounds. The aluminum hydride:tertiary amine compound remains dissolved in the ether while the 1:1 addition compound of lithium aluminum hydride and tertiary amine precipitates. Separation of the solid from the solution, for example, by filtering, yields the solid addition product which can be thermally decomposed to form substantially pure lithium aluminum hydride free from aluminum hydride.

We have further found that the ether soluble aluminum hydride:tertiary amine addition compound reacts with lithium hydride to form the same lithium aluminum hydride:tertiary amine addition compound as prepared from these two components directly. A further embodiment of our invention, therefore, in the preparation of pure lithium aluminum hydride comprises reacting lithium hydride with the aluminum hydride:tertiary amine addition compound to form further quantities of the lithium aluminum hydride:tertiary amine addition compound and its dissociation into purified lithium aluminum hydride. The conversion may be accomplished by the addition of lithium hydrideto the ether solution of the aluminum hydride:tertiary amine addition compound which may contain or have been filtered from the insoluble lithium aluminum hydride:tertiary amine addition compound. Further quantities of the insoluble addition compound are thus precipitated and may be separated and decomposed to form pure lithium aluminum hydride.

Because the aluminum hydride:tertiary amine addition compound is. appreciably volatile, a modified procedure is also advantageous. The ether extract of impure lithium aluminum hydride containing aluminum hydride is treated by the introduction of the tertiary amine and the excess amine is removed by distillation through a fractionating column containing lumps of lithium hydride as part of the packing. The volatile aluminum hydride: tertiary amine compound in contact with the lithium hydride in the fractionating column is converted to the lithium aluminum hydride:tertiary amine compound and the distillate of tertiary amine is free of aluminum hydride. Ether is subsequently removed by fractionation leaving pure lithium aluminum hydride as a residue.

The fol-lowing examples illustrate in detail various embodiments falling within the broad scope of our invention.

Example I manner: Six grams of commercial lithium aluminum hydride was powdered and was then transferred to .a tube of 50 ml. capacity. This tube was fitted with a /3 taper (inner member) which was attached to a vacuum apparatus and was evacuated. To prevent the fine lithium aluminum hydride powder from being swept into the vacuum system, a plug of glass wool was inserted into the neck of the taper before attachment to the vacuum system. After the evacuation had been completed, two gaseous liters (4.8 grams) of the commercial trimethylamine was condensed into the tube using a 63.5 C. bath and the mixture of lithium aluminum hydride and trimethylamine was maintained for 2 /2 hoursat this temperature. After .this time, the cooling bath was removed and the mixture was permitted to stand overnightat room temperature. Any non-condensable gases which were formed as a result of chemical reaction between the lithium aluminum hydride and the impurities associated with the trimethylamine were separated from the mixtureof excess lithium aluminum hydride and the solid .lzl .mole addition product of lithium aluminum hydride and trimethylaminepresent by maintaining the tube at liquid nitrogen .temperaturefand evacuating to a pressure of aboutlO- mm. Hg. The tube was, warmed to room temperature at "a "pressure of about 10- mm. Hg, the volatile.fproducts being condensed in a trap .kept at .l96 C.. The lithium aluminum hydride addition product was easily decomposed, yielding trimethylamine contaminated'with minor. proportionsof the aluminum hydride complex. 1 The trimethylamine was separated from the small amount of :trimethylamine-aluminum hydride complex by fractionation and collected as a liquid in a 763.9 'C. trap.. Recoveryof purified trimethylamine was about 90 to 95 percent of the starting material. It had a vapor pressure of685 mm. of mercury at C. compared with a literature value of 680.5 mm. Addition of diborane 'producedno hydrogen indicating the absence of water.

Exainplell 2 2% arcbm m lithium aluminum hydride of about 38fpercent'purity by weight and containing considerable amounts of aluminum hydride as an impurity were powdered and placed reaction flask 1 of the drawing which had been firstiflush'ed with dry nitrogen. Anhydrous diethyl ether (about 300 ml. or 215 grams) was thenintroduced into the flask through stopcock'2' from bulb 3. Any gases which were evolved during this operation were permitted to bubble 'oflf througha mercury bubbler 4. Stirring off 'the solution was accomplished by means of a small laboratory motor and mercury sealed stirrer. 5. The ether solution ,ot the aluminum hydride and lithiumaluminum' hydride was drawn ofi finto reaction bulb 6 through'a lo mmflcoarse fritted disc 7 by opening stopcock 3. There remained on the fritted disc various. impurities'iound 1in the mixture of the 'hydri'd'es. The mixtureof the two hydrides in reaction bulb 6 was then cooled to 0 Cfand anhydrous trimethylamine was added in liquid form through the condensingchamber 9 until precipitation appeared 'tobe complete. The condensing chamber consisted of an inner tubeof' 20 mm. tubing with an outer 'jacket o f 35mm. tubing. It was maintained at a low temperature by placing solid carbon dioxide inthe outer jacket. Thesolution in bulb 6 was stirred continuously during the addition of the trimethyl amine by means of a magnetic stirrer anda glass sealed stirring bar 10 placed in the reaction bulb before beginning the reaction. After the precipitation of the 1:1 molar addition product of the lithium aluminum hydride and trimethylamine was complete, the reaction flask 6 and receiver .11 .wererotated through an arc of 180, thus allowing'fthefs'olution to filter through a 30' coarse I fritted .,disc 'f1 2. Th h'er insolublelithium aluminum 'hydride-trimethylamine additionpro'duct was dissociated by. pumping the trimethylamineotf atlabout. 10- mm. Hg pressuretlirough a'trfificbol'ed by liquidfnitrogen to condense the. 'trimethylaminejwhile the; complex being dissociated vfv'as'f'maintainedat roomtemperature The lithium aluminum hydridefre'maining "in flask 6' was of better than 99" percent purity. The yield was "about 5 or about ,60 percent of 'the'jlithiumialuminum' hydride contained jin1the starting" material. flThe yield can be "improved byfmorethorough ether. extraction; Aparticularly "poor' same "of aluminum hydride was intentionally selected for this example in order to demonstrate the advantage of the method.

Example III Two 50 ml. round-bottom flasks with standard and the system was evacuated. Anhydrous diethyl etherml.) was introduced onto the lithium aluminum hydride by means of "aside 'tube on the flask and a liquid nitrogen bath surrounding the flask. The evacuated systern was closed and warmed to room temperature where it was maintained for'several hours with occasional shaking. The solution was filtered through the fritted glass plate by tilting the apparatus. Excess trimethylaminewas passed through the filtrate until precipitation appeared complete. The mixture was filtered and the solid freed fromether by evacuation at low ipressure (10" mm. Hg) and low temperature ('=l96 "C.). Trimethylamine was determined by vacua'tionat room temperature and condensation of the liquid at liquid nitrogen temperature.

The residual lithium aluminum hydride was analyzed for aluminum -and hydrogen by known methods. The solid addition compound of lithium aluminum hydrideand trimethlyamine contained 1.47 millimoles of Al, 5.76 millimoles of H, and 1.42 millimoles of trimethylamine, corresponding to molar ratios of l:3.92:0.96. Another sample similarly prepared was analyzed for lithium, aluminum and hydrogen. The molar ratios were 1 :1.01:3.91. Hence,'the formula for-the addition compound is LiAlI-I (CH N.

Example IV A fractionation column packed with lithium hydride pebbles was constructed. A diethyl ether solution of the aluminum hydride-trimethylamine addition compound was fractionated in the solur'nn to obtain trimethylamine of approximately percentpurity at the top of the column (no active'hydrogen) and a solution of lithium alur'ninumhydride in ether of-approximately 92 percent purity '(based on hydrogen and lithium analysis) at the bottom'of the column. Notrimethylamine was evident in the latter product.

Various modifications can be made in :the procedure of the specific eXamples-to'provide other embodiments which fall within thebroad scope of our invention. Thus, where it is desired to purify the lower tertiary alkyl amine, such tertiary alkyl'amine can be, for example, one in which each alkyl group contains from 1 :to 3 carb on atoms, such as 'trime'thylamine, dimethyl ethyl amine, "methyl diethyl amine, tri-n-propyl amine, dimethyl-n-propyl amineand 'the like, as well as the trimethyl'ainine'utilized in Example I. 1 Such tertiary amines can have associated with 'them one or moreimpurities such as water, ammonia, primary alkyl amines, or secondaryalkyl amines to'thee'xtent of about '10 percent by weight and where the impurities are amines :they will have from 1 to 3 carbon atomsin each alkylgroup, for example, monome'thyl amine, dimethyl amine,-r'n'ono 'ethyl' amine, diethyl amine, monoisopropyl amine, diisopropyl "amine, methyl isopr'opyl amine, etc.

Where our invention is employed'for the purposeof purifying lithium aluminumhydride containingaluminumf hydride as an impurity, the amount of aluminum hydride-present will generally be-within the range of from about /z to about lQper'cent by weight, depending iiponthe precise fashionin which the lithiumialuminum hydride has been prepared. -In place'of'the trim ethylamin employed in Example II it is also possible to use other tertiary alkyl-amines containing'Tr'om l to 3 carbon atoms in each-'alkylradical, forexample,

triethyl amine, tri-n-propyl amine, methyl diethylainine,

ethyl dimethyl amine, dimethyl-n-propyl amine, etc. In place of the diethyl ether used in Example II there can be added other dialkyl ethers containing from 1 to 3 carbon atoms in each alkyl radical such as dimethyl ether, methyl ethyl ether, methyl n-propyl ether, di-npropyl ether and the like.

We claim:

1. A method for the preparation of the 1:1 molar addition product of lithium aluminum hydride and a lower tertiary alkyl amine in solid form which comprises reacting equimolar amounts of lithium aluminum hydride and a lower tertiary alkyl amine thereby forming a 1:1 molar addition product of the lithium aluminum hydride and the lowertertiary alkyl amine in solid form.

2. A method according to claim 1 in which the lower tertiary alkyl amine is trimethyl amine.

3. A 1:1 molar addition product of lithium aluminum hydride and a lower tertiary alkyl amine, said addition product being in solid form.

4. A 1:1 molar addition product of lithium aluminum hydride and trimethylamine, said addition product being in solid form.

5. A method for the separation of a lower tertiary alkyl amine from a mixture containing the same and as an impurity a small amount of at least one material selected from the group consisting of water, ammonia,

lower primary alkyl amines and lower secondary alkyl amines which comprises reacting the mixture of the lower tertiary alkyl amine and impurities with a molar amount of lithium aluminum hydride, based on the moles of lower tertiary alkyl amine present, thereby forming the 1:1 molar addition product of the lower tertiary alkyl amine and the lithium aluminum hydride.

6. A method according to claim 5 in which said addition product is decomposed under a vacuum to separate its lower tertiary amine component from its lithium aluminum hydride component.

7. A method according to claim 5 in which said lower tertiary alkyl amine is trimethyl amine.

8. A method for the preparation of a 1:1 molar addition product of lithium aluminum hydride and a lower tertiary alkyl amine which comprises reacting lithium hydride and a 1:1 molar addition product of aluminum hydride and a lower tertiary alkyl amine.

9. A method according to claim 8 in which said amine is trimethyl amine.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD FOR THE PREPARATION OF THE 1:1 MOLAR ADDITION PRODUCT OF LITHIUM ALUMINUM HYDRIDE AND A LOWER TERTIARY ALKYL AMINE IN SOLID FORM WHICH COMPRISES REACTING EQUIMOLAR AMOUNTS OF LITHIUM ALUMINUM HYDRIDE AND A LOWER TERTIARY ALKYL AMINE THEREBY FORMING A 1:1 MOLAR ADDITION PRODUCT OF THE LITHIUM ALUMINUM HYDRIDE AND THE LOWER TERTIARY ALKYL AMINE IN SOLID FORM. 